This invention relates to a secondary battery having a non-aqueous electrolyte. More particularly, it relates to a secondary battery containing a non-aqueous electrolyte therein, in which a lithium compound oxide and a carbonaceous material are used as a positive electrode and as a negative electrode, respectively.
Recently, new types of electronic equipment, such as a VTR with a built-in camera, portable phone or a lap-top computer, which are designed with an increasingly smaller size and weight, have made their debut. In keeping up therewith, a secondary battery has come into the limelight as being a portable electric power source, and searches and investigation are now proceeding briskly for realization of a higher energy density secondary battery.
Thus a lithium ion secondary battery has been proposed as a secondary battery having an energy density higher than that of a secondary battery containing an aqueous electrolyte, such as a lead battery or a nickel-cadmium battery, and is being put to practical application.
Meanwhile, since a low-viscosity solvent, employed for an organic electrolyte for a lithium secondary battery, gives only a low charging/discharging efficiency, if used alone, it is mandatory that such low viscosity solvent be mixed with a high dielectric constant solvent, such as propylene carbonate (PC). Besides, mixing PC into the low viscosity solvent leads to an advantage that the electrical conductivity higher than that obtained with the sole component system consisting only of the low viscosity solvent may be achieved. For this reason, a mixed solvent consisting of a mixture of PC and 1,2-dimethoxyethane (DHE) has so far been employed for the organic electrolyte for the lithium secondary battery.
However, the secondary battery having the non-aqueous electrolyte, in which the mixture of PC and DME is used as the solvent, has a disadvantage that, since DME has a low melting point, a current breaker sensitive to an internal pressure is erroneously activated to disable charging/discharging if the surrounding working temperature is raised to about 100.degree. C. For this reason, such battery cannot be employed in a car in summertime. Besides, since DME is poor in stability with respect to the compound oxides of lithium, the battery capacity is deteriorated irrevocably if the battery is stored in the charged state.
As for low viscosity solvents other than DME, a low molecular weight carbonic acid ester having 1 to 3 number n of carbon atoms has been proposed in JP Patent KOKAI Application No. 2-148665, in which it is stated that the cycle life of the negative electrode of lithium metal may be improved by employing such carbonic acid ester alone as an organic solvent for the lithium secondary battery. However, in such case, the cycle life of the battery is on the order of 250 cycles which is considerably lesser than that of existing nickel-cadmium secondary battery which is about 500. Thus the lithium secondary battery cannot not be said to be unobjectionable as a battery for practical utilization.
On the other hand, if a mixture of PC with the above-mentioned low molecular weight carbonate ester is employed, the cycle life characteristics may be improved. Above all, with a secondary battery having the non-aqueous electrolyte, in which the carbonaceous material is employed as the negative electrode, a cycle life exceeding 1,000 cycles may be achieved. For example, with a mixed solvent composed of PC and a high boiling diethyl carbonate (DEC), among low molecular carbonic acid esters, an increase in the battery internal pressure is less likely to be incurred due to rise in the ambient temperature than in the case of employing a mixed solvent composed of DME and PC, resulting in improved stability with respect to the positive electrode. However, such battery suffers from a disadvantage that the capacity retention ratio, represented by the ratio of the capacity before the charging/discharging cycle to that after the charging/discharging cycle, is lower, with the result that the battery cycle life is shortened.
Researches have also been conducted on a mixed sol vent composed of PC with dimethyl carbonate (DMC). It has been confirmed that the use of the mixed solvent leads to improved stability with respect to the positive electrode and the prolonged cycle life. However, the mixed solvent composed of DMC and PC has a drawback that it has a low boiling point and a high solidifying point thus leading to a narrow range of the battery working temperature.
In sum, the conventional secondary battery having the non-aqueous electrolyte has suffered from an inconvenience that, since the organic solvent is not fully suitable, limitations are placed on the working temperature, while a sufficiently long cycle life has not been achieved.